In order to regulate vehicular traffic in an efficient manner, intersections with traffic signals often include an inductance loop vehicle detection system that senses the presence of vehicles located at an intersection. The basic configuration of such a vehicle detection system includes an induction loop (e.g., a wire coil) that is located beneath a roadway. The induction loop is connected to a control box that includes one or more oscillators, an oscillation counter, and a traffic signal controller. The oscillator is used to produce an oscillating signal in the induction loop and the oscillation counter stores information relating to the frequency of the oscillation in the induction loop.
When a vehicle approaches an intersection with a vehicle detection system, conducting material (e.g. metal) in the vehicle decreases the inductance of the induction loop, thereby increasing the oscillation frequency of the detection system. The decrease in inductance occurs because the conducting material in the vehicle acts as a shortened turn in the induction loop. To determine whether a vehicle is present at the intersection, the vehicle detection system compares the current oscillation frequency with a prior oscillation frequency. If the current oscillation frequency is greater than the prior oscillation frequency, a vehicle may be present. If, however, the current oscillation frequency is less than or equal to the prior oscillation frequency, then it is likely that a vehicle is not present at the intersection.
Between successive oscillation counts, a vehicle may enter an intersection, remain at an intersection, or leave an intersection. If the oscillation counter senses an increase in oscillation frequency, a control signal is generated in order to alert the traffic signal controller of the presence of a vehicle. Generally, the traffic signal controller will respond to the control signal after a preset delay period by changing the traffic signal configuration (e.g., from a red traffic light signal to a green traffic light signal). If the traffic signal controller has not altered the traffic signal configuration and a succeeding oscillation count remains at the higher level, the detection system generates a control signal to alert the traffic signal controller of the continued presence of a vehicle. Similarly, if a succeeding oscillation count returns to a base level, the detection system generates a control signal to alert the traffic signal controller of the vehicle's departure, thereby eliminating the need for a change in the traffic signal's configuration.
In addition to vehicles that are located proximal to the inductance loop, environmental factors may affect the inductance of the loop. In multi-lane intersections, vehicles stopped in one lane may alter the inductance of a loop located in a neighboring lane. Where intersections are located near train tracks, the presence of a train may also affect loop inductance. Further, fluctuations in electrical load may alter the inductance properties of the loop. Accordingly, inductance loop vehicle detection systems are often calibrated such that small changes in oscillation frequency do not generate a signal that affects the traffic signal configuration. Only changes in frequency that are above a preset level will affect the operation of the traffic signal controller.
A common, detrimental effect of the calibration is that vehicles with a relatively small quantity of conducting material, including bicycles, motorcycles, and smaller automobiles, do not cause an inductance sufficient to alter the oscillation frequency of commonly used vehicle detection systems beyond the calibrated level. When such a vehicle enters an intersection with an inductance loop detection system, the detection system does not generate a control signal to alert the traffic signal controller of the vehicle's presence. Accordingly, there is a need in the art for an apparatus and/or method that may be used in conjunction with vehicles that have a relatively small quantity of conducting material to activate inductance loop vehicle detection systems.